Worm Breeder's Gazette 9(2): 92

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

A Maternal Effect Mutation That Results in XO-Specific Lethality

J. Plenefisch and B. Meyer

I.  Mutations in genes involved in the process of dosage 
compensation, sex determination, and/or the assessment of the X 
chromosome to autosome ratio often adversely affect only XO(XY) or XX 
animals.  In Caenorhabditis s in five genes (
dpy-21, dpy-27, 
ferential expression in XX 
animals, whereas there is no visible phenotype in XO animals.  This 
has been interpreted as a disruption of dosage compensation, leading 
to a general increase of X-linked gene expression in XX animals.  
Biochemical evidence suggests that this interpretation is valid, at 
least for the genes dpy-21, py-28 and egl-16 (WBG 
v. 9 n. 1: Meyer and Casson, Villeneuve and Meyer).
We have identified a new mutation resulting in XO-specific lethality.
This maternal-effect mutation, y2, is located on chromosome II 
between the markers unc-85 and dpy-10 (but is not under the deficiency 
mnDf30).  It was isolated in a general screen for sex specific lethals 
(WBG v. 8 n. 3 ) .  Briefly, hermaphrodites homozygous for him5(e1490) 
were mutagenized using EMS and descendant F2s were cloned onto 
individual plates.  Their broods were scored for evidence of sex 
specific lethality; a y2 brood devoid of males was identified as a 
candidate for further study.
We observed that y2/y2 sons of y2/+ animals are both viable and 
functionally normal, but y2/y2 animals produced only daughters.  
Mutations with this phenotype could result from 1) inviability of XO 
animals, 2) suppression of him-5(e1490), or 3) sexual transformation 
of XO animals into hermaphrodites.  The possibility that y2 fails to 
produce nullo-X ova was eliminated by mating males bearing the X-
linked marker lin-14 with y2 him-5(e1490) hermaphrodites: equal 
numbers of lin-14 and wild-type males were generated.  Further 
evidence in support of y2 causing XO inviability rather than the 
failure to form XO eggs comes from progeny counts.y2; him-5(e1490) 
strains produce about 40% inviable offspring, primarily as dead eggs.  
These inviable zygotes can be accounted for by the normal distribution 
of 30% XO males and 10% dead eggs associated with him-5(e1490).  (
There is also some inviability and infertility among hermaphrodite 
progeny of y2 (non-him) animals.)
The evidence that y2 fails to transform XO animals into 
hermaphrodites is based on the failure to produce Unc non-Rol [XO] 
hermaphrodites in crosses of y2/+ males with y2 rol-6(e187); 
1) hermaphrodites.  In addition, only half the 
expected number of male progeny were produced, consistent y2/y2 being 
a XO specific lethal.
Finally we showed, by constructing double mutants with the sex 
transformer genes tra-1(e1099 and e1488), tra-2(e1095), and her-1(
n695sd), that this lethality is karyotype rather than sex specific; 
y2/y2 XX pseudomale offspring of y2/y2 mothers are viable.  We also 
noted that y2 had no morphological effects on the degree of 
transformation of these mutants, suggesting that the effects of y2 on 
the sex determination pathway are minimal.
We were interested in exploring whether the bias toward XO lethality 
is associated with a reduction in the expression of X-linked genes in 
y2 animals.  Using genetic assays developed by L.  De long (lin14) and 
P.  Meneely (lin-15) we have shown that this mutation results in the 
hypo-expression of at least these two X-linked loci.  By these assays 
the reduced X expression in y2 is a simple zygotic phenotype of both 
XO and XX animals.  We have also put y2 in combination with mutations 
in dpy-21, ulting strains are 
inviable, contrary to our naive expectation that simple rescue of y2 
by these mutations (or vice versa) might occur.  Additionally, 2 
copies of the X-chromosome duplications mnDp25 and mnDp10 fail to 
restore XO viability.  We are presently determining the relative 
levels of X-linked gene expression in these strains.
Additional alleles of the the gene defined by y2 as well as 
suppressors of this mutation are being sought to aid our analysis of 
this gene.
II.  The mutation her-1(n695sd) causes an incomplete masculinization 
of XX animals.  Mutations in the chauvinistic dpy genes (dpy-21, 
dpy-27, n the partial 
suppression of the transformed tail phenotype of her-1(n695sd).  We 
felt that a clonal reversion screen of n695 would give us a method of 
identifying new mutations in genes involved in the process of dosage 
compensation.  We isolated 12 candidates from 6000 F2 clones.  Two of 
these suppress not only the tail transformation of n695 but also 
restore the HSNs (and hence egg-laying ability) to these animals.  
Both are extragenic, one mapping to chromosome I and one to III; 
neither appear to have a visible phenotype independent of n695.  These 
mutations appear specific for n695, as they do not suppress egl-41, 
another gene which displays a weak Tra phenotype.  egl-16(n485) does 
not block the suppression of n695 by these mutations, which can be 
contrasted to the fact that egl-16 does block the suppression of n695 
by dpy-21(e428) (Villeneuve, personal communication).
We also isolated several enhancers of n695 transformation, which are 
in the process of being characterized.